The present disclosure relates in general to semiconductor manufacturing technology, and more particularly, to a system and method for improving accuracy of critical dimension metrology in a semiconductor wafer processing. The present disclosure also relates to a method and system for improving accuracy of critical dimension metrology through providing an adjustable refractive index/extinction coefficient (n/k) of thin films in the substrate during regression modeling of scatterometry-based optical critical dimension (OCD) metrology.
In integrated circuit manufacturing technology, a resist layer is typically applied to a semiconductor wafer surface, followed by an exposure of the resist through a mask (e.g., a reticle or photomask). A post-exposure bake is then performed to initiate the deprotection reaction (for the positive-tone resist), which makes the chemically amplified resist in the exposed area more soluble in the developer and thus will be developed away in the subsequent development process. An after-development inspection (ADI) is then performed to inspect the critical dimension (CD) and profile of the exposed resist using a scanning electron microscope (SEM) to determine whether it conforms to a specification. If the resist is within specification, a pattern is etched or transferred and the resist is stripped. An after-etching inspection (AEI) is then performed on the wafer.
Traditional SEM inspection, however, becomes a bottleneck for providing accurate and repeatable CD due to the ArF resist being easily damaged by the e-beam and the inherent line edge roughness of the resist pattern. In response, a scatterometry-based optical critical dimension (OCD) metrology is often used instead of SEM inspection. OCD employs light mainly in the visible range for CD measurement, thus has little impact to the measured resist pattern. In addition, since only patterns of large gratings (about 50 μm squared) are measured and only light scattered to a specific angle is collected, OCD is impervious to line edge roughness. Further, OCD can detect not only the CD but profile (manifested as side-wall angle (SWA)) of the resist pattern as well as thickness of each thin film in the wafer substrate. Thus, OCD provides more consistent and comprehensive CD information than SEM.
Both SEM and OCD may be used in after-development inspection and after-etching inspection to optimize CD uniformity. With existing SEM or OCD tools, inter-field CD uniformity may be optimized. Inter-field CD uniformity optimization may be obtained by examining the field-to-field CD difference between a plurality of fields on a wafer. For example, inter-field CD uniformity optimization may be performed over 80 fields to improve the quality of selected measurement points of a wafer surface area. In addition, intra-field CD uniformity optimization may be performed with existing SEM or OCD tools by examining CD differences within a field of the wafer.
However, unlike SEM tools, OCD tools obtain CD measurements indirectly and entirely based on the resist profile reconstructed through simulation. Thus, the accuracy of CD and side-wide angle (SWA) measurements rely entirely on the accuracy of the simulation. In order to reduce computation load, many OCD tools simulate the resist profile by assuming a fixed refractive index/extinction coefficient (n/k) of each thin film in the substrate. For example, the refractive index/extinction coefficient (n/k) of a thin film in the substrate, such as organic bottom anti-reflection coating (BARC), is fixed during regression modeling of the OCD metrology.
Other types of thin films, such as inorganic BARC, may have varying refractive indices/extinction coefficients (n/k) due to a difference in control of the thin film deposition process. Thus, the assumption of fixed refractive indices/extinction coefficients (n/k) may cause OCD errors for these types of thin films. Therefore, a need exists for a method and system that improve accuracy of OCD metrology by varying the refractive indices/extinction coefficients (n/k), such that OCD errors due to fixed refractive indices/extinction coefficients (n/k) may be minimized.